This happens regardless of the device I'm using, whether I use WiFi or Cellular network. I've reset the account, reset sync, used the "clean database" function in advanced settings, and reinstalled and set up PI, to no avail. I still get this error.

I've also tried waiting between sync attempts. The last wait was at least 6-7 hours - and still got the error message.

I have no errors syncing with other services (Google Calendar) or applications (Todo, Toodledo Native App) - only PI.

The issue is actually with Toodledo. They changed their sync method prior to informing developers. PI and others are working to get the sync issue resolved and expect an application update (not bug fix since it was a Toodledo change...) within 1-2 weeks.